The aims of this study are to determine 1) if PTH can prevent hypogonadism induced bone loss in women, 2) to investigate the roles of ovarian hormones on PTH and vitamin D metabolism. In this proposal the interactions of ovarian hormones and parathyroid hormone (PTH) in the regulation of bone metabolism will be investigated in young, adult women and rats. First, women will endometriosis will receive a long-acting GhRH-analogue to induce reversible estrogen deficiency. Half of these patients will also receive daily hPTH-(1-34) injections to investigate the effects of chronic PTH administration on vitamin D metabilism and bone mass and to determine whether PTH therapy can prevent hypogonadism-induced bone loss. The remaining patients will serve as controls and will also be used to define the effects of gonadal steroid deficiency on PTH and vitamin D metabolism using state-of-the-art techniques. Second, young menopausal women will receive IV calcium or PTH injections before and during estrogen replacement in order to examine the effects of estrogen on clcium regulitory hormones.